In pulsed return echo transmit/receiver systems, such as radar or sonar systems, for example, the pulsed signals from a transmitter pulsed signal source may vary in their amplitude, frequency and/or phase characteristics from pulse to pulse so that the received target echo pulses are to some degree dissimilar, that is, they contain distortions which arise as a result. It is desirable to reduce the effects of such distortions as much as possible.
An example of a system in which such distortions are undesirable is a radar system which is used to develop transmitter pulse signals requiring the utilization of a high power pulsed signal source, such as a magnetron, for example. Magnetrons are relatively inexpensive and have a long life and tend to be very reliable in such a context and so their use is highly desired in many applications. However, unlike coherent power sources, such as klystrons, magnetrons can generate relatively severe amplitude, frequency and/or phase distortions.
In many such radars, however, a further problem which arises in the signal processing of the incoming target return, or echo pulse, signals in the radar receiver, is that such return pulses are often accompanied by ground clutter; i.e., returned echo pulses from fixed targets on the ground which tend to mask or otherwise obscure the returned pulses from the desired moving target which may be at the same range. One technique for removing, or at least substantially reducing, the effects of ground clutter is to utilize Doppler processing or Doppler filtering techniques, radars using such techniques being known in the art as Doppler filter or moving target indicator (MTI) radars. For such a Doppler processor radar system to operate effectively, however, the high powered transmission pulses must be substantially identical to one another and any deviations or distortions in the pulse amplitude, frequency or phase characteristics thereof tend to prevent the Doppler system from effectively reducing the ground clutter effects so that the system becomes less useful.
Magnetrons, however, tend to produce pulses whose characteristics are not exactly the same from pulse to pulse. In many cases the differences are sufficient that their amplitude, frequency and/or phase variations introduce problems in processing the return echos. Such magnetrons are in effect pulsed oscillators, the phase of the output pulse is substantially completely random on a pulse to pulse basis and, in addition, there may also be small amplitude and frequency modulation effects which are introduced from one pulse to another.
One technique for compensating for phase variations in such radar systems is to use a coherent oscillator circuit, sometimes referred to as a "coho circuit", a technique that has been long used in MTI radars. While variations from pulse to pulse of the average phase can be relatively effectively compensated by the use of such circuits, the remaining amplitude modulation (AM) and frequency modulation (FM) effects often still limit the system's performance. Normally, it has been found that the FM effect is the most difficult to compensate for and is often the more critical of the two modulation effects. One approach to avoiding the problem of AM and FM distortions has been to design relatively expensive magnetron structures having very low AM and FM distortions. Such designs are very difficult to implement and tend to measurably increase the cost of the power tubes and the transmitter portion of the system, particularly when an overall radar system has already been installed, is in use, and must be retrofitted with such newly designed magnetrons. It would be more desirable to avoid such costly redesign and to devise receiving techniques which use appropriate circuitry for compensating for such AM and FM effects. This is particularly so if such circuitry can be used in existing systems to retrofit such systems in such a way as to improve their performances at a reasonable cost.
Another proposed technique includes the use of a transversal equalizer circuit in which a received, or return echo, signal in its complex form, i.e., a form having both in phase and quadrature components, is sampled in time. The digitized samples are then compared with the transmitted pulse, also in its complex form. The circuit recognizes changes in the structure of the transmitted pulses, on a pulse to pulse basis, and compensates for amplitude, frequency and phase changes. After compensation, the waveforms of the received pulses approach those which would have been received if the transmitter had time-invariant amplitude and phase characteristics. More specifically, such circuitry includes a tapped delay line to which received signal samples are supplied, the delayed output from each of the taps being weighted in accordance with a suitable weight generator circuit and the weighted signals being appropriately summed to supply an output supply to the receiver. The weights as determined by the weight generator circuit are discretely changed from transmitted pulse to transmitted pulse in accordance with suitable selected criteria so that the output to the receiver is appropriately compensated for with respect to amplitude, frequency and/or phase variations in the transmitted pulse signals prior to the supplying of the corresponding received echo signals to the receiver circuitry.
While the latter technique achieves a reasonable degree of effectiveness in some applications, such compensation approach does not provide a sufficient improvement in the received signal for many other applications and it is desirable to devise an even better way of compensating for such distortion effects.